Controlling the weather

The weather was absolutely shocking in the summer of 1952. After weeks of rain in the west country, Exmoor was saturated until, on August 15 an awesome nine inches of rain fell in just 12 hours. The water cascaded down valleys in a raging torrent that eventually tore through the coastal town of Lynmouth, ripping down buildings and killing 34 people, the worst British flashflood in living memory.

But something sinister might also have played a part. Declassified papers now reveal that secret rainmaking experiments by the military had been taking place on Exmoor. Aircraft showered clouds with silver iodide, a dusty powder just the right size for water droplets to grow on. We will probably never know for sure if the experiment ran out of control, or whether the freak rains were going to fall in any case.

That has not stopped the military being mesmerised by the idea of turning the weather into a weapon. During the Vietnam war, the Americans launched Project Popeye, a secret mission to seed the tops of monsoon clouds and trigger phenomenal downpours that would wash away the Ho Chi Minh Trail used for ferrying supplies.

For five years Vietnam, Cambodia and Laos were sprayed during the monsoons, and military intelligence claimed that rainfall was increased by a third in some places. It only came to an end in March 1971 when journalist Jack Anderson exposed the project and caused such a public furore that the UN general assembly approved a universal treaty banning environmental warfare.

But the US air force planners recently came up with new proposals to launch new weather weapons. Instead of silver-iodide, the idea is to shower fine particles of heat-absorbing carbon over clouds to trigger localised flooding and bog down troops and their equipment. Lasers on aircraft would also trigger lightning onto enemy aircraft, whilst other lasers could be fired at fog to clear a path over enemy targets on the ground.

Whether or not they work, past experiences tell us to be wary of tampering with the weather. In 1947, meteorologists tried to kill off a dying hurricane out at sea by seeding the clouds. The following day, the hurricane suddenly gathered strength, swung round and hit Savannah, Georgia causing extensive damage. The weather boffins were so rattled by the disaster it was not until August 1969 that they dared try again.

When Hurricane Debbie was 700 miles out at sea, they flew three seeding missions around its eye, where tropical storms are at their most intense, but the results were mixed - with each seeding the hurricane's winds were reduced and each time they picked up again.

Today, many experts argue that we should not be tinkering at all with tropical storms because they help cool the world by pumping surplus heat from the earth's surface out into space like giant safety valves. And there lies the paradox of trying to change almost all weather - we are not really sure if it works, and if it does work we do not know what other forces we might unwittingly unleash.